This invention relates generally to tunable optical transmitters, and more particularly to prediction methods for tunable semiconductor lasers for stabilizing such lasers and for maintaining a high side mode suppression ratio (SMSR).
Dense wavelength division multiplexed (DWDM) optical networks increase the information carrying capacity of a communication (e.g., transmission) system by loading multiple channels, each at a different carrier frequency or wavelength, onto a single optical fiber. It is advantageous in such systems to use optical sources (or transmitters) that can produce a spectrally narrow output at any one of a subset of the desired channel wavelengths. Distributed Bragg reflector (DBR) lasers are often used in DWDM systems because they typically have spectrally narrow outputs.
The output wavelength of a tunable DBR depends in part on the value of a tuning current. Typically the spectral output comprises a series of wavelength steps where the output wavelength remains relatively constant at each step for a finite range of tuning currents. However, as these systems are operated for long periods of time, DBR semiconductor lasers tend to degrade in performance due to aging and material defects. For single frequency semiconductor lasers the aging behavior is relatively well characterized and manifests itself in the form of reduced power output, and alteration of the output wavelength produced at a particular operating current. As a result, the output wavelength of semiconductor lasers may drift as the device ages.
Age induced wavelength drift may result in mode hopping where the device output abruptly switches to a different longitudinal mode. In a DWDM system, a channel experiencing a mode hop abruptly starts to operate in a mode (i.e., at a carrier wavelength) different from that originally assigned (e.g., at a channel wavelength different from that designated by an ITU grid point). Mode hopping may be avoided through closed loop wavelength feedback, which adjusts the value of the tuning current upon detection of the onset of a mode hop.
Additional closed loop wavelength feedback may also be used to provide low level side-band emissions by maintaining the tuning current near the center of the appropriate current step. However, in conventional systems, there is no guarantee that the beginning of life (BOL) values for any of the set points and control inputs of the device will be the correct values to use as the device ages. The result of this may be a xe2x80x9cswitchxe2x80x9d to a channel other than the one desired by the system designer.
Therefore aging algorithms are often utilized to predict the operating currents of devices operating in the field. For a tunable laser, it is desirable to operate at the center of a tuning step. Inaccurate aging predictions that provide a tuning current near the edges of the appropriate current step may result in poor sidemode suppression and possibly an unintended mode hop. Accurate (but imperfect) predictions will still give an operating point with good sidemode suppression, but may lead to reduced performance for parameters like chirp and RIN.
In one aspect of the present invention a method for compensating for age induced changes in the tuning characteristic (referred to hereafter as wavelength drift) in a semiconductor laser includes constructing an aging matrix relating the change in th e tuning characteristic associated with one channel""s desired operating point to the change in the tuning characteristic associated with another channel""s desired operating point as a function of factory aging of the semiconductor laser. This aging matrix may be stored on a laser controller for future use. The controller may also monitor the change of the in field value of the tuning characteristic associated with the first output channel (a local channel). The controller may then calculate the value of the tuning characteristic for a non-local channel in accordance with the aging matrix, the amount of local channel aging, and the uncompensated tuning current for the non-local channel.
In another aspect of the present invention a method for operating a tunable DBR includes determining an aging vector in accordance with a ratio of change in a tuning characteristic associated with a non-local channel and change in tuning characteristic associated with a local channel as a function of factory aging of the tunable DBR, measuring the in field value of the tuning characteristic associated with transmission at the local channel and updating value of tuning characteristic to switch to the non-local channel in accordance with the measured in field value of the tuning characteristic associated with transmission at the local channel and said aging vector.
In another aspect of the present invention an apparatus includes a DBR transmitting at a local channel, a measurement apparatus for measuring the value of an in field tuning characteristic associated with the local channel and a controller, coupled to the DBR for switching the output wavelength of the DBR in accordance with an aging matrix relating the change in a tuning characteristic associated with the local channel to the change in the tuning characteristic associated with the non-local channel as a function of the factory aging of the DBR and the value of the in field tuning characteristic associated with the first output wavelength.